arrFiles = new Array('btnHome', 'btnAbout', 'btnProducts', 'btnContact');
arrImg = new Array();

// preload the on-mouseover images
for(i = 0; i < arrFiles.length; i++) {
  arrImg[arrFiles[i]] = new Image();
  arrImg[arrFiles[i]].src = 'img/' + arrFiles[i] + '_on.jpg';
}

function mnuOver() {
  el = window.event.srcElement;
  el.src = arrImg[el.id].src;
}


function mnuOut() {
  el = window.event.srcElement;
  el.src =  'img/' + el.id + '.jpg';
}